Sorry to hear about your losses and this might sound like lecturing but I think it's worth saying.
What price did you pay for those businesses? When you say they had solid fundamentals what does that mean? A pretty common mistake is to overpay for a business with good potential. Unless you have some unique insight everyone already knows the business has potential and it's priced that way. The market as a whole had crazy crazy multiples which means it was overpriced even including the growth prospects. Sure, if you think Tesla can get to a point where it's selling all the cars in the world then you definitely should buy that stock at the price it was trading at.
Overpaying on margin is just compounding your problems. I never ever buy stocks on margin (and generally I avoid borrowing money for anything but the most solid investment, like buying a house). You have to always think about the worse case scenario and be willing to live with it. Ofcourse gambling a lot of money can lead to making a lot of money- it's just that the expected value is negative.
The other thing you always need to consider is how the company you're investing in will perform in an economic downturn. Recessions aren't an if, they're a when. There's a certain chance of recession every year. If you believe the company has strong enough fundamentals to survive a recession and strong enough management/leadership to steer it through difficult times then just hold on. Presumably you have a mix of those so on the aggregate you should do ok. If these great companies are below book price then double down on them but keep in mind the market is disagreeing with your evaluation. Otherwise you've miscalculated the expected value of your investments i.e. your belief in the companies and their fundamentals was incorrect.